The invention relates to an optical transmission network, comprising an optical transmission medium with one or more optical fibers, to which, at one side, one or more first optical transceivers are connected and to which, at the other side, one or more second transceivers are connected, said first transceivers each having a first transmitter and a first receiver and said second transceivers each having a second transmitter and a second receiver, a first transmission signal being transmitted by the first transmitter of a first transceiver and being received by the second receiver of a second transceiver, and a second transmission signal being transmitted by the second transmitter of that second transceiver and being received by the first receiver of that first transceiver.
Lasers for optical communications have the property that their optical frequency suffers from fluctuations and drift. In a coherent point-to-point connection the receiver usually is tuned to the transmitting frequency of the transmitter. The receiver follows the (opposite) transmitter by means of automatic frequency control (AFC).
In a coherent network with many transmitters and receivers, distributed all over the network, a rather complex solution will be necessary. Some different solutions within this field are known:
With a scanning spectrum analyser (for instance with a Fabry-Perot interferometer or a wavemeter) the frequencies of the several transmitter lasers are locked; the spectrum analyser itself may be stabilized by means of an atomic or molecular spectrum line. A disadvantage of this method is that all laser signals have to be able to be measured and controlled at one (central) location [1].
All optical signals are commonly put through one multifrequency passfilter (for instance a ring resonator or a Fabry-Perot filter), the various laser frequencies being set at the various pass frequencies of that passfilter. This solution has the same disadvantage as the former solution, viz. the said central controllability of the lasers in use [2].
All optical signals are locked to a common reference signal, for instance a frequency comb signal, generated by a comb generator. This solution requires extra optical hardware for distribution of the comb signal.
The frequency of each separate laser may be locked to an atomic or molecular resonance frequency (spectral line). A serious disadvantage is that only rather few resonance frequencies are available and besides this method is rather complex, inflexible and expensive [3].
The distances between the laser frequencies could be chosen such that any collision will be excluded, but this method would imply a very inefficient use of the optical bandwidth and besides the tuning range of lasers is limited.
Several combinations and variants of the above methods are known.